1. Field of the Invention
The present invention relates to a particle detecting method and a storage medium storing a program for implementing the method, and more particularly to a particle detecting method of detecting the number of particles carried by a gas stream.
2. Description of the Related Art
In general, in a substrate processing apparatus having a processing chamber in which desired processing is carried out on a semiconductor wafer (hereinafter referred to as “the wafer”) as a substrate using a process gas, particles, such as metal particles of aluminum, are produced due to contact between the wafer and a mounting stage on which the wafer is placed, and reaction products, such as fluorocarbon polymer particles, are produced due to reaction of the process gas.
These particles are deposited on the wafer, causing degradation of the quality of a semiconductor device formed on a surface of the wafer. To avoid this problem, in the substrate processing apparatus, the particle diameters and number of particles within the processing chamber are detected so as to maintain the particle diameters and the particle count below respective predetermined values. When the particle diameters and the particle count become larger than the respective predetermined values, operation of the substrate processing apparatus is stopped for cleaning of the processing chamber and replacement of component parts.
As a method of detecting particles of the above-mentioned kinds, a method has conventionally been known in which scattered light generated due to the presence of particles is measured by a particle monitor provided in an intermediate portion of a purged gas flow path through which particles and gases are evacuated from the processing chamber.
In this method in which scattered light is measured, a light beam (light flux) formed in a sheet shape (belt shape) is passed through a gas stream flowing through the purged gas flow path, and the intensity of a scattered light generated when a particle contained in the gas stream passes through the light beam is measured by a sensor disposed in facing relation to the purged gas flow path, whereafter the particle diameter of the particle is calculated based on the measured scattered light intensity (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2000-146819).
Particles each pass by in front of the sensor as time elapses. For this reason, as indicated by the values of scattered light intensity associated with respective particles Pf and Ps shown in FIG. 7, scattered light intensity measured by the sensor progressively increases with the lapse of time at first, and then progressively decreases after having reached its extreme value. To detect the particle diameter of each particle accurately, it is preferred that associated scattered light intensity is continuously measured with the lapse of time. In this case, however, the amount of measurement data becomes immense, and therefore it inevitably takes a long time to process the data. Further, changes in scattered light intensity may be approximated using the Gaussian curve based on a plurality of pieces of measurement data, but also in this case, curve fitting requires time.
To solve this problem, in recent years, a detection method has been employed in which a measuring time period is divided into measurement periods each defined by a predetermined time period, and the scattered light intensity is measured (discretely) at predetermined time intervals in the measurement periods (T1 to T5 in FIG. 7). In this detection method, during each measurement period, the maximum value of scattered light intensity during the measurement period is selected, and stored in a memory or the like. Further, if the selected maximum value of scattered light intensity exceeds a predetermined threshold value, it is determined that a single particle is passing, and the particle diameter of the particle having passed is calculated based on the maximum value of scattered light intensity. According to this detection method, since only one maximum value of scattered light intensity is selected and stored during each measurement period, it is possible to reduce the amount of data, thereby shortening a time period required for data processing.
Further, according to this detection method, insofar as a particle, such as the particle Pf in FIG. 7, which passes by in front of the sensor within a single predetermined period (T1) is concerned, only one maximum value of scattered light intensity PfI is selected, so that it is possible to accurately measure the number of particles having passed by in front of the sensor.
In the above described detection method, however, insofar as a particle, such as a particle Ps in FIG. 7, which passes by in front of the sensor over a plurality of predetermined periods (T2 to T5), i.e. a low-speed particle is concerned, four maximum values Pfl1 to Pfl4 of scattered light intensity in the respective associated periods T2 to T5 are selected, and therefore even though the single particle Ps has passed by in front of the sensor in actuality, it is erroneously determined that four particles at the maximum have passed by in front of the sensor. In short, the number of low-speed particles cannot be detected accurately.
If the number of particles cannot be detected accurately, unnecessary cleaning of the processing chamber or unnecessary replacement of component parts might be performed, resulting in degradation of the operating efficiency of the substrate processing apparatus.